1. Field of the Invention
The present invention relates to an ink set, and an image forming apparatus and method, and more particularly, to an ink set including two liquids, and an image forming apparatus and method using this ink set.
2. Description of the Related Art
With the rapid progress of inkjet technology and digital cameras and other digital imaging technology in recent years, it has become possible to obtain high-quality photographic prints which surpass silver halide photographs, easily, even in a general domestic situation. On the other hand, inkjet technology has started to be adapted for and applied to many diverse fields, such as industrial printing. However, in the field of industrial printing, it is essential to achieve high-speed operation, from the viewpoint of productivity, but there have been no ink and image recording method which enables high-quality prints of the level of a silver halide print to be produced at high speed. Therefore, research has been started into various recording methods for providing images of high quality at high speed, and one of these methods involves the use of a responsive ink set constituted of a combination of a treatment liquid and an ink liquid. This responsive ink set enables the bleeding of ink (ink bleeding) to be reduced.
Furthermore, an image forming apparatus has been known which adopts a so-called intermediate transfer system, in which an image is formed on an intermediate transfer body, and the image formed on the intermediate transfer body is then transferred to a recording medium. In the image forming apparatus of intermediate transfer type, it has been difficult to achieve high quality on the intermediate transfer body as well as achieving good transfer characteristics. This is because a liquid flow is created on the intermediate transfer body and the image quality is liable to decline as a result of this flow. In view of these circumstances, various methods have been proposed in order to achieve good image quality on the intermediate transfer body.
Japanese Patent Application Publication No. 2001-30616 and Japanese Patent Application Publication No. 2002-52804 disclose polymer particles which tend to be subjected to a reaction with polyvalent metal salt contained in a treatment liquid, and which have film forming properties, whereby high-resolution and high-quality images can be obtained. Japanese Patent Application Publication No. 2003-82265 discloses a method in which coloring material and polymer particles are aggregated by means of a polyvalent metal salt, which is similar to the one disclosed in Japanese Patent Application Publication Nos. 2001-30616 and 2002-52804, or a poly allylamine, and thereby an image is formed on an intermediate transfer body. In this method, the aggregating force of the coloring material aggregate is increased. Furthermore, Japanese Patent Application Publication No. 2006-83277 discloses an ink containing polymer particles which are self-dispersible in water and whose the diameter varies with the characteristics (the pH or the conductivity) of the aqueous solution. The aggregation of coloring material in this ink is promoted by means of the aggregation of the polymer particles due to a change in the pH of the ink.
However, although Japanese Patent Application Publication No. 2001-30616 and Japanese Patent Application Publication No. 2002-52804 provide effective methods for obtaining an image with high-resolution and high-quality up to a certain degree, they are problematic in that the ink bleeding is liable to occur in image border sections, and that density non-uniformities are liable to occur in the case of a solid image. The reason for these problems are considered to be as follows. If the aggregating force of the polyvalent metal salt is too strong, then in the initial stage of reaction, an aggregate of the pigment (coloring material) and polymer particles is formed only at the contact interface between the treatment liquid and the ink. This aggregate formed at the contact interface then prevents the pigment (coloring material) and the polymer particles contained in the ink from diffusing into the treatment liquid, and at the same time, prevents the polyvalent metal salt contained in the treatment liquid from diffusing into the ink, resulting in the nonuniform progress of the reaction within an image. Consequently, there exist portions where the aggregating action acts too strongly and portions where the reaction is insufficient.
The technology disclosed in Japanese Patent Application Publication No. 2003-82265 is effective, when an image of high image density in which the ink bleeding is restricted to some extent is formed by intermediate transfer method on the recording medium which is a permeable paper such as a normal paper. However, since this technology utilizes the aggregating action similar to the one as in Japanese Patent Application Publication Nos. 2001-30616 and 2002-52804, then non-uniformities of the reaction occur in the image forming region, and therefore variations in the aggregating force are liable to arise. Consequently, transfer non-uniformities and transfer defects occur, and there is a possibility that the quality of the image formed on the recording medium will be seriously impaired. Furthermore, since unreacted coloring material is liable to remain, then the coloring material flows (moves) on the transfer body, and the image quality at the stage of forming an image on the intermediate transfer body cannot be regarded as adequate.
Moreover, Japanese Patent Application Publication No. 2006-83277 indeed has the effect of promoting the aggregation of the coloring material due to the aggregating effect of the polymer particles as a result of the change in the pH of the ink, but, depending on the form (or extent of aggregation) of aggregation of the polymer particles, a problem may arise in that satisfactory effects of achieving high image quality such as effects of the suppression of the ink bleeding or the suppression of density non-uniformities, or the like cannot be obtained.